Ophiomitrella, Verrill, 1899

Thuy, Ben, 2013, Temporary expansion to shelf depths rather than an onshore-offshore trend: the shallow-water rise and demise of the modern deep-sea brittle star family Ophiacanthidae (Echinodermata: Ophiuroidea), European Journal of Taxonomy 48, pp. 1-242 : 179-181

Ophiomitrella ? sp. 2

Fig. 32 : 1

Material examined

GZG.INV.78740 (1 dissociated LAP) from sample 89, Jhurio Formation, middle Bathonian, Middle Jurassic of Jumara, India.

Description

GZG.INV.78740 is a dissociated, small, median to distal LAP; nearly as high as wide; dorsal edge very weakly concave as a result of a constriction; distal edge convex; proximal edge concave, devoid of spurs; outer surface partly obscured by sediment; probably with coarsely meshed stereom displaying irregularly thickened trabeculae. Six nearly equal-sized, equi-distant, ear-shaped spine articulations freestanding on elevated distal portion of LAP; proximally sharply bordered by exceptionally thick, straight, strongly prominent ridge; gap between spine articulations and distal edge of LAP very narrow. Ventral edge of LAP weakly convex, no tentacle notch discernible.

Inner side of LAP largely obscured by sediment; inner side of distal edge devoid of spurs; inner side of tentacle notch very small.

Remarks

The single, poorly preserved specimen described above shares greatest similarities with the median to distal LAPs of at least some of the extant species of Ophiomitrella , in particular with respect to the conspicuous, unusually thick ridge proximally bordering the spine articulations. This assignment, however, is far from robust, and the absence of better-preserved material precludes both a more secure and precise taxonomic assessment and a comparison with the above-described, slightly younger LAPs from Germany, here tentatively assigned to Ophiomitrella .

Note

The articulated specimen from the upper Maastrichtian Gronsveld Member (Maastricht Formation) of Maastricht, the Netherlands, assigned by Jagt (2000) to Ophiacantha ? danica, most probably represents an undescribed species of Ophiomitrella . Corresponding dissociated LAPs ( Fig. 32 : 2) found in adjacent beds corroborate such an assignment. This new species of Ophiomitrella is here only mentioned as such; it will be described fully in a future study.

Figures

Fig. 32. Fossil skeletal plates of ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1. Ophiomitrella? sp. 2 from the Bathonian (Middle Jurassic) of Jumara, India; GZG. INV.78740, median to distal LAP. 2. Ophiomitrella sp. nov. from the late Maastrichtian (Late Cretaceous) of Maastricht, the Netherlands; NHMM JJ 5104, proximal LAP. 3-6. Ophiomalleus beneficarum gen. et sp. nov. from the Callovian of Bauer-Wehrland (3), Althüttendorf (4-5) and Hohensaaten (6), Germany. 3. GZG.INV.78741 (holotype), proximal to median LAP. 4. GZG.INV.78742 (paratype), proximal LAP. 5. GZG.INV.78743 (paratype), distal LAP. 6. GZG.INV.78744 (paratype), arm spine. 7-9. Ophiomalleus stevenwilsoni gen. et sp. nov. from the latest Hauterivian (Early Cretaceous) of Sarstedt, Germany. 7. GZG.INV.78748 (holotype), proximal LAP. 8. GZG.INV.78749 (paratype), median LAP. 9. GZG. INV.78750 (paratype), distal LAP. One common scale bar per species except for 6.